1. Field of the Invention
This invention relates to photoactive crosslinking compounds prepared by reacting an electrophilic 2-alkenyl azlactone compound and a nucleophilic aromatic ketone. These photoactive compounds can be copolymerized with acrylic monomers and photoactivated by actinic radiation so as to crosslink such polymers.
2. Background Information
Pressure sensitive adhesives (PSAs) made by photopolymerizing an alkyl acrylate and a polar copolymerizable monomer are known in the art. See, e.g., U.S. Pat. Nos. RE 24,906, 4,181,755, 4,364,972, and 4,243,500. Acrylic-based PSAs exhibit good adherence to high energy (i.e., polar) substrates.
Solvent-processed acrylic PSA compositions can be crosslinked by adding a polyfunctional crosslinking agent that reacts with a reactive group present in the polymer. See, e.g., Japanese Kokoku 58[1983]-046236 in which is described a solvent-processed crosslinked acrylic PSA wherein incorporated isocyanate groups are available for reaction with the crosslinking agent.
Hot melt coating a PSA composition eliminates the necessity of solvent processing. To hot melt process an adhesive composition, the composition must be uncrosslinked during the coating process; however, to achieve a PSA with balanced properties (i.e., peel and shear adhesion), the composition eventually must be crosslinked. In hot melt coating processes, this is usually done by exposure to high energy radiation (e.g., E-beam or high intensity ultraviolet radiation). Commonly, when high intensity ultraviolet radiation is used, a photoactive crosslinking species such as benzophenone is added to the composition.
A more efficient method of photocrosslinking involves incorporating mer units including pendent hydrogen abstracting moieties into the polymer backbone prior to coating. Such polymers can be hot melt coated and subsequently cured by conventional irradiation techniques. This process is typified by U.S. Pat. No. 4,737,599 where a PSA with good adhesion to skin is described.
The cohesive strength of an acrylic PSA can be increased without unduly affecting its compliance by utilizing a photoactive crosslinking agent in conjunction with a photoinitiator. See, e.g., U.S. Pat. Nos. 4,181,752, 4,329,384, 4,330,590, 4,391,687, and 5,202,361. Useful photoactive crosslinking agents include various aldehydes, quinones, and particularly certain chromophore-substituted halomethyl-s-triazines (because they provide desirably shortened reaction times and somewhat greater tolerance to oxygen over the non-halomethyl-containing agents), although their use can result in evolution of HCl.
Copolymerizable photoinitiators such as 2-[4-(2-hydroxy-2,2-dimethyl-1-oxopropyl)phenoxy]ethyl 2-propenoate and their use in the polymerization of ethylenically unsaturated compounds is disclosed in U.S. Pat. No. 4,922,004.
Japanese Kokai 2[1990]-248482 describes a photocurable PSA obtained by reacting (a) 30 to 50 parts by weight (pbw) of a copolymer of an acrylic acid alkyl ester, a copolymerizable ethylenically unsaturated monomer having a polar group, and a copolymerizable monomer with a photosensitizing group (such as 2-acryloyloxybenzophenone or 1-acryloyloxy-2-[4-(4-chlorobenzoyl)benzoyloxy]ethane); (b) 40 to 60 pbw of an aryloxy acrylic monomer such as phenoxyethyl acrylate or nonylphenoxyethyl acrylate; and (c) a tackifying resin. The composition is cured using a total dose of energy of 300 to 800 mJ/cm.sup.2 from a high pressure mercury lamp. Such high intensity ultraviolet radiation is likely to produce an adhesive that has a shear strength value less than 100 minutes.
Similarly, DE 43 03 183 C1 (Germany) discloses a method for producing PSA layers comprising the steps of thickening a monomer mixture that includes a photoinitiator with a separately made solvent-free saturated UV-reactive polyacrylate, coating the thickened mixture onto a substrate, and irradiating the coated substrate. The separately made polymer comprises side chains that, when irradiated, participate in crosslinking reactions. The sole example involves the addition of a commercially available polymer having a molecular weight of about 200,000 to a monomer mixture that is then polymerized.
The shear values of PSAs prepared by actinically irradiating acrylic monomers can be enhanced by the addition of polyacrylic crosslinking agents. See, e.g., U.S. Pat. No. 4,379,201. Such PSAs involve networks and are sensitive to processing conditions.
An ultraviolet (UV) radiation-curable composition that includes a copolymer of ethylenically unsaturated monomers, ethylenically unsaturated monomers, and optionally one or more polyethylenically unsaturated compounds is described in U.S. Pat. No. 5,180,756.
When attempting to photocrosslink acrylic PSA compositions, one of two broad categories of photoactive crosslinking agents is generally used: an .alpha.-cleaving agent or a hydrogen abstracting agent. Of the latter category, the most commonly used example is probably acryloylbenzophenone (ABP). This photocrosslinker is an efficient crosslinker, but it is not always soluble in the relatively non-polar monomers that make up PSA monomer formulations.
Acrylic derivatives of anthraquinone, benzophenone, xanthone, thioxanthone, and 9-fluorenone have been described previously, as has an acrylamide derivative of anthraquinone. However, none of these compounds has been described as being useful as a reactive crosslinker for PSA compositions.
What has not been previously described is an easily prepared, effective hydrogen abstracting-type photocrosslinking agent that exhibits enhanced solubility in relatively non-polar monomers.